1. Field of the Invention
The present invention relates to a magnetic recording/reproducing apparatus using a current-perpendicular-to-plain magnetoresistive head.
2. Description of the Related Art
An increase in recording density is always required in a magnetic recording/reproducing apparatus. Accordingly, a high-sensitive read head is required. As an element for a read head that satisfies such requirements, a tunneling magnetoresistive element (TMR element) or a current-perpendicular-to-plain giant magnetoresistive element (CPP-GMR element) are known.
In a read head using such an element, a sense current for magnetic field detection is flowed through a TMR film or a GMR film, having a stacked structure of a magnetization pinned layer, a spacer layer and a magnetization free layer, in a direction substantially perpendicular to the film plane. For this reason, the read head can be called a current-perpendicular-to-plain magnetoresistive head. Examples of documents that disclose a magnetic head using a TMR element include, for example, U.S. Pat. No. 5,898,548. Examples of documents that disclose a magnetic head using a CPP-GMR element include, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-55512 and U.S. Pat. No. 5,668,688.
When a sense current is flowed through a read head element, use of constant-current driving or constant-voltage driving is determined depending on various conditions. For example, it is known that the constant-voltage driving is preferred for a read head using a TMR element in order to absorb fluctuation in element resistance.
The spacer layer in the TMR film or the GMR film, having the stacked structure of the magnetization pinned layer, the spacer layer and the magnetization free layer described above, has a structure in which fine conductive regions (referred to as pin holes or metal holes) are distributed in a high-resistance matrix. The reason why the spacer layer having such a structure is employed is to control the resistance of the spacer layer. The resistance of a high-resistance tunnel barrier layer as the spacer layer in the TMR element must be suppressed in consideration of practical use as a read head. The fine conductive regions in the tunnel barrier layer contribute to the decrease in resistance (see, for example, U.S. Pat. No. 5,898,548, and IEEE Trans. Magn., Vol. 38, 2002, p. 73). In a CPP-GMR element, a high-resistance thin oxide film including fine metal holes is used as the spacer layer so as to improve an MR ratio.
However, it has been found that these current-perpendicular-to-plain magnetoresistive heads may be deteriorated mainly due to heat generated by sense current concentrated on the fine conductive regions distributed in the spacer layer, resulting in shortened life-time thereof, which brings about a problem of lower reliability of a magnetic recording/reproducing apparatus.